The structure of a typical cellular wireless communication system is described next while referring to the concept diagram in FIG. 1. In a cellular wireless system, the wireless base stations 1201 cover an area called a cell. The cell may be called by names such as a target cell; microcell, serving cell, or femtocell according to the width of that coverage range and type of function. The cell providing the largest coverage area for the cellular wireless base station 1201 is also called a target cell base station and may accommodate mobile stations 1203 within an area of approximately several hundred to several thousand meters. The base station 1201 is capable of communication with IP networks or connecting “calls” between mobile stations 1203 at positions separated from each other, by way of a core network 1202 made up of upstream devices such as the base station control device, etc.
Third generation cellular wireless communication systems using the CDMA (Code Division Multiple Access) method can communicate multimedia information by way of an IP network. Wireless systems utilizing CDMA include for example UMTS (Universal Mobile Telecommunications System) and EVDO (Evolution Data Optimized). The enriched mobile contents in this environment have created greater demand in the form of needs for higher speed broadband communication services. Support for wideband communications in forms such as best effort type, voice communication by VoIP, and content distribution such as video via streaming information is expected to come into general use.
Wireless communication systems that utilize OFDMA (Orthogonal Frequency Division Multiple Access) method are the focus of much attention, as next generation wireless communication systems to replace CDMA in view of their wide-bandwidth and high-speed. The OFDMA method utilizes a hardware configuration better suited for wide bandwidths than the CDMA method and is better able to provide large capacity wireless communications.
The OFDMA method is technology that improves bandwidth resource utilization efficiency by multiplexing multiple orthogonal carrier waves (frequency sub-carriers) onto the frequency bandwidth. Cellular wireless communication systems utilizing the OFDMA method are classified as beyond 3G (generation) cellular wireless communication systems and typical standards include E-UTRA (Evolved-UMTS Terrestrial Radio Access). This E-UTRA also called LTE (Long Term Evolution) has been established as an international standard by the 3GPP (3rd Generation Partnership Project) which is a standardization group. The title E-UTRA indicates a wireless access method, while E-UTRAN (Evolved-UMTS Radio Access Network) indicates an access network.
Radio frequency interference from adjacent base stations and from terminals grouped under those base stations is known to effect communications during wireless access that connects terminals and base stations in wireless communication systems. Interference from adjacent base stations and terminals grouped under those base stations is known to be especially large in OFDMA wireless communication systems, and base station installation design is a highly difficult task.
During wireless access at cell edges which are boundaries between base station coverage areas (separation between sectors and cells, etc.) the desired signal level from the cell itself must compete with the interference power level from neighboring cells. These rival signal levels cause a drastic deterioration in SINR (Signal-to-Interference-and-Noise-Power-Ratio) which is an index indicating channel quality.
In the overall system, reducing the interference between neighboring cells fulfills an important function in improving channel capacity during wireless access. Reducing interference between neighboring cells mainly focuses on lowering the transmit power of all wireless stations to the minimum required level. However, simply suppressing the power level will cause problems such as shrinking the coverage area or lowering the communication speed. The excess power must therefore first be specified before lowering the power level.
Controlling interference during wireless access uplinks heavily involves curbing the transmit power of terminals in the cell edge. Terminals located at the cell edge must overcome the transmission path loss that occurs between the base stations while also maintaining a specified communication quality and so need high power during transmissions. A method disclosed in JP 2009-159637 was therefore proposed, that gives selection priority to cell management by base stations Whose terminals have small transmission path loss.
Suppressing the transmission power while maintaining a specified communication quality requires allocating more bandwidth resources than usual and lowering the rate for encoding error correcting symbols. Lowering the power will reduce interference but increases the load during wireless accessing due to greater use of bandwidth resources.
Besides lowering interference during wireless access as described above, load dispersion control between cells is required at locations where terminal connections are concentrated. If the load exceeds the processing capacity of the base station backbone then there is a high probability that call connections will be difficult to establish.
Approaches have been studied for achieving load dispersal by active use of handovers to neighboring cells. Handover algorithms in wireless systems must comply with the standard specified entity and procedures.